Acta Polytechnica CTU Proceedings doi:10.14311/APP.2018.18.0044 Acta Polytechnica CTU Proceedings 18:44–47, 2018 © Czech Technical University in Prague, 2018 available online at http://ojs.cvut.cz/ojs/index.php/app IMPACT TESTING OF ORDNANCE GELATINE UNDER MODERATE STRAIN RATE CONDITIONS Tomáš Doktor∗, Petr Zlámal, Jan Šleichrt, Tomáš Fíla, Daniel Kytýř Czech Technical University in Prague, Faculty of Transportation Sciences, Konviktská 20, Praha 1, Czech republic ∗ corresponding author: doktor@fd.cvut.cz Abstract. An experimental study on energy absorption capabilities and strain rate sensitivity of ordnance gelatine was performed. Strain energy density under quasi static compression and moderate strain rate impact tests was compared. In the study two types of material were tested, bulk ordnance gelatine and polymeric open-cell meshwork filled with ordnance gelatine. From the results a significant strain-rate effect was observed in terms of ultimate compressive strength and strain energy density. In comparison of the deformation behaviour under quasi static conditions and drop weight test the difference was very significant, however slight increase in both strength and strain energy density was observed even between different impact energies and velocities during the impact testing. The peak acceleration was significantly reduced in polymer meshwork filled by gelatine in comparison to the bulk gelatine. Keywords: ordnance gelatine, strain energy density, impact testing. 1. Introduction In the engingeering applications in the field of ballistic and blast protection, passive safety systems in vehi- cles (automotive, railway vehicles) the impact energy dissipation is among the crucial properties [1]. One of the possible solutions to achieve such capabilities is to utilise cellular solids in conjunction with strain rate sensitive filling [2]. In this study experimental investigation on strain rate sensitivity at moderate strain rates as well as the interaction of the cellular solid and viscous filling is described. As filling mate- rial ordnance gelatine was selected due to a flawless preparation procedure and availability. The ordnance (ballistic) gelatine is widely used as tissue simulant for assessment of damage of tissues induced by bullets [3]. However strain rate sensitivity under compressive load- ing was reported as well [4]. Deformation response of ballistic gelatine blocks (plain and reinforced with polymeric meshwork) under mod- erate impact loading is described in this paper in terms of peak acceleration during the impact as well as total kinetic energy dissipation. 2. Materials and methods 2.1. Sample preparation For both impact and quasi-static testing two types of material were used, (i) bulk ordnance gelatine and (ii) open-cell polymeric meshwork filled with ordnance gelatine. The gelatine was prepared according to the reports of Jussila [5]. As a base, 260 Bloom beef gelatine (REMI MB, Ltd., Czech republic) was used. The gelatine powder was poured into warmed water (temperature was 45 °C) In the second group the gelatine was reinforced with polymeric open-cell meshwork FT-S10FR (Foam Techniques, Ltd, United Kingdom) with tetrakaideca- hedral cells, mean cell size 6 mm, mean strut thickness 0.6 mm. The dissolved gelatine after homogeneous mixing was poured into the meshwork. The samples from both batches (both pure and reinforced) were cured for 24 h in room temperature and subsequently stored for 24 h in refrigerator. After this curing the blocks were cut into samples of desired size. In the quasi static loading conditions the size of samples was 25 × 25 × 50 mm, and samples’ size for impact testing was approximately 60 × 50 × 50 mm. Dimensions of the samples were limited by diameter of used loading plates. To ensure a better focus during optical observation captured face of each sample was sprayed granit paint to obtain random pattern. 2.2. Quasi static tests The quasi static tests were performed using electrome- chanic uniaxial loading device Instron 6530 (Instron, Inc., USA). The compressive tests were displacement driven with loading rate 3 mm · min−1, which corre- sponded to strain rate 0.001 s−1. Maximum displace- ment was set to 30 mm. 2.3. Impact tests instrumentation The moderate strain rate compression tests were per- formed by an in-house drop tower developed at CTU FTS [6]. The drop weight is released by electromag- netic member and then guided on steel rods and induces the impact on a sample places at a stable plate. The drop-tower setup was instrumented by tri- axial accelerometer (EGCS3, TE Connectivity, Ltd., 44 http://dx.doi.org/10.14311/APP.2018.18.0044 http://ojs.cvut.cz/ojs/index.php/app vol. 18/2018 Impact Testing of Ordnance Gelatine Figure 1. Drop tower setup (1 - motorized drop- weight lift, 2 - trigger inductive sensor, 3 - guide rods, 4 - electromagnetic release member, 5 - drop-weight guide frame, 6 - accelerometer, 7 - drop-weight). USA) with loading capacity ±1000g and impact force transducer (200C20, PCB Piezoelectronics, USA) with loading capacity 89 kN. Both accelerometer and force transducer were connected to read out electronics NI9234 (National Instruments, Inc., USA) providing sampling rate 51200 samples per second. For the ob- servation of deformation process a high speed camera (IDT NX3, USA) was employed. The camera provided frame rate 2895 to 3310 fps at resolution 768 × 1312 px and the exposure time varied from 89 to 115 µs. For illumination of the loading scene pair of flash illumina- tors Veritas Constellation 60 were used. The impact test setup is depicted in Figure 1 and Figure 2. 2.4. Impact test procedure The impact tests were carried out in three arrange- ments with different initial height and mass of the impactor. In each arrangement a pair of samples from the group A (bulk gelatine) as well as from group B (polymer meshwork filled with gelatin) were tested. The initial heights and impactor masses are listed in Table 1 with corresponding impact energy, velocity and strain rate. The impact velocity was calculated using the initial heigt and gravity constant based on conservation of sum of kinetic and potential energy. Figure 2. Drop tower setup (1 - specimen, 2 - im- pactor, 3 - flash-illumination, 4 - high-speed camera). 0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 tr u e s tr e s s [ M P a ] true strain [-] Stress - strain diagrams at quasi static compression A, 0.001/s B, 0.001/s Figure 3. Stress-strain curves of quasi-static tests. 3. Results From the quasi static loading test stress-strain curves which are depicted in Figure 3 were calculated based on the measured force F and cross-head displacement u by formulae σeng = F A0 and �eng = u l0 , where A0 is specimen cross-section and l0 is the initial gauge length. The engingeering stress σeng and strain �eng were then converted to logarithmic strain � and true stress σ using formulae � = ln�eng and σ = σeng(1 + �eng). To evaluate the drop tests stress-strain diagrams were created from the measured acceleration and force. While the stress is measured directly by force trans- ducer, strain calculation is based on double time in- tegration of the vertical component of acceleration. 45 T. Doktor, P. Zlámal, J. Šleichrt et al. Acta Polytechnica CTU Proceedings arrangement initial height drop weight impact velocity impact energy [m] [kg] [ms-1] [J] 1 1.00 4.495 4.4 44.1 2 1.50 4.495 5.4 66.1 3 1.50 6.504 5.4 95.7 Table 1. Impact test parameters. 0 2 4 6 8 10 12 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 tr u e s tr e s s [ M P a ] true strain [-] Stress - strain diagrams group A 73/s, 44.1 J 73/s, 44.1 J 90/s, 66.1 J 90/s, 66.1 J 90/s, 95.7 J 90/s, 95.7 J Figure 4. Stress-strain curves of drop tests of group A (ordnance gelatine). 0 1 2 3 4 5 6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 tr u e s tr e s s [ M P a ] true strain [-] Stress - strain diagrams group B 88/s, 44.1 J 88/s, 44.1 J 108/s, 66.1 J 108/s, 66.1 J 108/s, 95.7 J 108/s, 95.7 J Figure 5. Stress-strain curves of drop tests of group B (polymer meshwork filled with ordnance gelatine). Comparison of the stress-strain curves is depicted in Figure 5. For the assessment of dissipated impact energy strain energy density λ was computed using formula λ = ∫ �max 0 σ d� The obtained values are presented in Table 2. Moreover for visual evaluation of the deformation be- haviour the captured loading scene was used (selected series of loading scenes in distinct loading steps is depicted in Figure 6). No. / strain energy peak group density [Jcm-3] acceleration 1 / A1 1.04503 234.92g 2 / A1 1.00956 209.23g 3 / B1 0.57674 95.01g 4 / B1 0.74435 94.07g 5 / A2 1.12309 210.53g 6 / A2 0.98409 231.10g 7 / B2 0.55441 149.72g 8 / B2 0.79479 147.84g 9 / A3 1.42620 199.30g 10 / A3 1.69909 223.24g 11 / B3 1.22701 152.42g 12 / B3 1.24369 148.73g Table 2. Impact test results. 4. Conclusions An experimental study on deformation behaviour of ordnance gelatine and polymeric open-cell meshwork. From the results a significant strain-rate effect was observed in terms of ultimate compressive strength and strain energy density. In comparison of the defor- mation behaviour under quasi static conditions and drop weight test the difference was very significant, however slight increase in both strength and strain energy density was observed even between different impact energies and velocities during the impact test- ing. The comparison between tested types of materials shows a significant reduction of the peak acceleration during the impact while the strain energy density decreased only slightly. This behaviour may be ex- plained by the flow of the filling material thru the meshwork during the impact. The obtained results will provide a base for further modelling of the impact energy dissipation as well as for optimisation of the filling material used in cellular solids. Acknowledgements The research was supported by the Czech Science Foundation (research project No. 15-15480S) and by 46 vol. 18/2018 Impact Testing of Ordnance Gelatine Figure 6. Loading scene of selected sample (group A3) in several loading steps (time step between captured images is 0.89ms). Czech technical university in Prague (research projects SGS17/148/OHK2/2T/16 and SGS18/155/OHK2/2T/16). All the support is gratefully acknowledged. References [1] P. Qiao, M. Yang, F. Bobaru. Impact mechanics and high-energy absorbing materials: Review. Journal of Aerospace Engineering 21(4):235–248, 2008. doi:10.1061/(ASCE)0893-1321(2008)21:4(235). [2] T. Doktor, P. Zlamal, T. Fila, et al. Properties of polymer-filled aluminium foams under moderate strain-rate loading conditions. Materiali in Tehnologije 49(4):597–600, 2015. doi:10.17222/mit.2014.195. [3] J. Breeze, M. Midwinter, D. Pope, et al. Developmental framework to validate future designs of ballistic neck protection. British Journal of Oral and Maxillofacial Surgery 51(1):47–51, 2013. doi:10.1016/j.bjoms.2012.03.001. [4] J. Kwon, G. Subhash. Compressive strain rate sensitivity of ballistic gelatin. Journal of Biomechanics 43(3):420–425, 2010. doi:10.1016/j.jbiomech.2009.10.008. [5] J. Jussila. Preparing ballistic gelatine - review and proposal for a standard method. Forensic Science International 141(2-3):91–98, 2004. doi:10.1016/j.forsciint.2003.11.036. [6] O. Jirousek, P. Zlamal, P. Koudelka, T. Fila. Development and validation of material models for closed-cell metal foam for impact simulation. Civil-Comp Proceedings 106, 2014. 47 http://dx.doi.org/10.1061/(ASCE)0893-1321(2008)21:4(235) http://dx.doi.org/10.17222/mit.2014.195 http://dx.doi.org/10.1016/j.bjoms.2012.03.001 http://dx.doi.org/10.1016/j.jbiomech.2009.10.008 http://dx.doi.org/10.1016/j.forsciint.2003.11.036 Acta Polytechnica CTU Proceedings 18:44–47, 2018 1 Introduction 2 Materials and methods 2.1 Sample preparation 2.2 Quasi static tests 2.3 Impact tests instrumentation 2.4 Impact test procedure 3 Results 4 Conclusions Acknowledgements References